40-year Johnstown flood anniversary: Could it happen again, and could it happen here?

Not too far from Happy Valley lies the city of Johnstown, known to many as the Flood City. Some don’t realize that the area was devastated by not one but three disastrous floods.

All three floods were sparked by weather, though only the widespread flood of 1936 was by weather alone. That flood was caused by the combination of heavy rainfall and a rapidly melting snowpack. It affected not only the Johnstown area, but still holds the high-water mark for many spots in the Pennsylvania. Both the deadly Great Flood of 1889 and the Flood of 1977 were sparked not only by rounds of heavy rainfall, but were enhanced by the failure of man-made dams. Following the 40th anniversary of the 1977 flood on July 20, let’s look at why the flood happened, if it could ever happen again and if it could ever happen to State College.

July 1977 was a hot month. Temperatures reached 90 degrees or hotter on eight days between July 6-22 in State College, and was even warmer in Johnstown. This heat wave was thanks to a massive ridge of high pressure that dominated the eastern United States. The Bermuda high nosed its way into the southeastern United States and helped to fuel our area with plenty of hot, extremely humid air. An upper-level disturbance formed a complex of thunderstorms along the top of this ridge. This disturbance and its showers and thunderstorms moved southeastward through the western into the south-central part of the state. On its own, this can bring flooding rain, but in this case we developed what is known as a training situation with the thunderstorms over the Johnstown region.

Training thunderstorms are when multiple thunderstorms form in a line parallel to the flow of the storms. They follow each other much like the cars of a freight train. Storm after storm bring rounds of heavy rainfall, and in this case, the thunderstorms were back building. This means a new thunderstorm would form on the backside of the line, which prolonged the rainfall for this event. Eventually the rainfall total exceeded 10 inches in less than a day near the Johnstown area.

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How localized was that event? Well, to put it in perspective, State College received less than an inch of rain. You take that amount of rainfall over higher terrain of the Laurel Highlands and the water rushes down to the lowest elevations overloading the local rivers and streams. This alone would have created disastrous flooding, but the amount of water also caused the failure of six dams in the region, much of which were built to prevent a repeat of the 1936 flood. These dam failures helped to send more than 100 million gallons of water down into the valley on top of what was coming from the heavy rain event. According to the National Oceanic and Atmospheric Administration and the Army Corp of Engineers, the previous channel improvements to move water through Johnstown were built to handle more than 81,000 cubic feet of water per second, but the combination of runoff and the dam bursts sent 115,000 cubic feet of water per second. This back up inundated downtown Johnstown.

All told, the flood caused 85 deaths and more than $200 million of damage. While this pales in comparison to the loss of more than 2,000 people lost in the 1889 flood, it was the last straw for many in the city. The population shrunk by more than 15 percent between 1970 and 1980, and while there were some other economic factors involved, the flood of 1977 shared a large blunt of the blame.

Could this happen again? Yes. The sharp terrain around the region makes it prone to flash flooding. And as long as there are dams that lead into the surrounding rivers, the potential will always be there for a dam burst. Though with modern inspections and engineering, we have the potential to minimize this risk.

Could this happen to State College? Somewhat. As you have noticed, the urbanization of the region has led to more problem spots with flooding. The more paved surfaces, the more runoff there is. Therefore, if we get hit with 10 inches of rain in less than a day, the flooding would be devastating; however, the broader valley in our region and the lack of major dams in higher elevations surrounding the immediate area helps a lot. So our risk is a lot smaller than the flood city.

If you would like to suggest a topic for Joe Murgo to address in this column, email him at Murgo@wtajtv.com.